I don't like my main entrance(s) to pass through soil. I like obsidian and planning out significant fortress features before ever digging into sub-surface rock.
The dwarves had to live and work predominantly out under the weather behind and on top of ugly silty clay walls sleeping (and being entombed) in increasingly cramped quarters constantly building stress and walls.
Ten years later the fruit of their (max 60 pop.) labor was a smoothed obsidian monolith with four entrances once again welcoming all migrants (pictured from a later date with equipment used to wage the arboreal war)
Each tower is intended to house a different death trap. The only 'completed' one makes magma mist. Bridges direct traffic from each of the four drawbridge entrances through the trap(s) or down towards the fort. Access to the courtyard is from deep within the fort.
A few levels below the four entrances converge where more bridges can re-direct invader flow back into the traps if needed.
From the center of the circle ramps lead a few more levels down to the welcome center. The bridges are over an 11-z drop but are more for re-locating corpse parts and death items out of the traders' sight and closer to their disposal zones then inflicting enemy damage.
A few more levels down the entrance passes through the main barracks which is sandwiched by the library and the tavern where foreigners are permitted.
The expense of the rough adamantine bridges is to mitigate the possible damage done by Fortress Defense Mod's hellfire imps' hellfire breath vaporizing bridges while the military is fighting on them.
Once the arboreal war has been won a ring of animal watch towers is planned.
